This invention relates to the purification of crude tetrahydrofuran prepared by the dehydrocyclization of 1,4-butandiol in the presence of an organic acid catalyst. The tetrahydrofuran, which forms an azeotropic mixture with water, is separated and purified to a high quality in a high yield by extractive distillation of the crude mixture.
Tetrahydrofuran (THF) has been widely used in various fields as a solvent for various organic compounds or high polymers and also as an intermediate of various organic compounds. Recently, THF has further found its use as a starting material or an auxiliary for synthetic polymers in textiles and plastics.
A number of methods have previously been suggested for the separation of THF from its water azeotrope. These generally consist of methods such as adding glycerine to a THF azeotrope whereby a layer separation occurs in which there is a THF-water layer containing more THF than the azeotrope which then can be distilled to yield anhydrous THF. The bottom layer contains more water than the azeotrope and can be concentrated to the azeotrope again by distillation. The use of a hydrocarbon, such as toluene or halo or nitro hydrocarbon of boiling point higher than THF has been disclosed for extraction.
Other methods of drying such materials were suggested, as adding a drying agent such as silica gel or calcium chloride or alumina or molecular sieves to the material to remove water. It is also known from U.S. Pat. No. 4,093,633 to produce anhydrous THF from its azeotropic mixtures by subjecting the mixtures to a two-step continuous distillation under two different pressures. U.S. Pat. No. 4,665,205 teaches that in the production of THF from 1,4-butanediol, 1,4-butanediol can be used as the extractive solvent to remove water from the reaction product and yield anhydrous THF.